Electrochemical conversion cells, commonly referred to as fuel cells, produce electrical energy by processing first and second reactants, e.g., through oxidation and reduction of hydrogen and oxygen. By way of illustration and not limitation, a typical polymer electrolyte fuel cell comprises a polymer membrane (e.g., a proton exchange membrane (PEM)) that is positioned between a pair of catalyst layers and a pair of gas diffusion media (GDM). The GDM typically includes a gas diffusion layer and a microporous layer (MPL). The cathode electrode layers can be part of the PEM (catalyst coated membrane (CCM)) or as a layer on the DM (catalyst coated diffusion media (CCDM)). A cathode plate and an anode plate (or bipolar plates BPP) are positioned at the outermost sides adjacent the gas diffusion media layers, and the preceding components are tightly compressed to form the cell unit.
The voltage provided by a single cell unit is typically too small for useful applications. Accordingly, large numbers of cells, typically about 200 up to about 300 or more, are arranged and connected in electrical series consecutively in a “stack” to increase the voltage electrical output of the electrochemical conversion assembly or fuel cell. Each cell typically includes a bipolar plate assembly (BPP) and a unitized electrode assembly (UEA) (GDM/Cat/PEM/Cat/GDM). The components must be assembled into the stack, resulting in about 400 to about 600 or more individual components to be assembled.
The UEA includes a subgasket material which acts to separate the anode and cathode flow streams and provides for electrical insulation between the anode and cathode plates of the fuel cell. There is a desire to reduce the thickness of the subgasket material to reduce costs and decrease membrane stress at the subgasket edge where the anode and cathode GDMs overlap the subgasket. Some current UEAs are made with thin film subgaskets, in the range of 1 to 2 mil thick, which are mechanically weak and can be difficult to handle. When the UEA with a thin subgasket is assembled into a fuel cell stack as a discrete component it can be difficult to obtain good positional alignment of the trimmed UEA profile features to the BPP features in the fuel cell stack assembly.
A seal is disposed around the perimeter of the cell and also between the reactant streams. The subgasket material of the UEA needs to be sealed between adjacent plate assemblies of the fuel cell as the subgasket film materials do not provide adequate sealing integrity to the plate materials on their own. Elastomer seal material typically provides this function. The seal material can be equally disposed on each side of the subgasket film, or the majority of the elastomer can exist on one side while the other side has only a thin layer of seal material (microseal). In some cases, the UEA is sealed to the BPP using a microseal of pressure sensitive adhesives (PSAs), which require a special release liner to prevent undesired or premature adhesion. It is desirable for the subgasket film material to extend to or slightly past the plate edges to act as an electrical insulator between the anode and cathode plates of a cell. PSAs present on these overhanging subgaskets can pose additional stack assembly issues as the adhesives are exposed and active in these areas where adhesive function is not necessarily desired.
Therefore there is a need for a cell assembly which is easy to handle and subsequently assemble into a stack, while also maintaining good dimensional alignment between all of the components and features that make up the stack assembly.